Semiconductor Detectors

Summary of Last Lecture

Band structure in Solids:

Conduction band

Valence band

Insulator

Eg > 5 eV

Conduction band

Valence band

Semiconductor

Eg ~ 2 eV

thermalconductivity:

ni=AT e

T absolute temperatureK Boltzmann constantA material constantEg band gap

-Eg/(2kT)3/2

room temperature:pure Si: 1.5 x 1010/cm3

pure Ge: 2.5 x 1013/cm3Charge carrier in conductor: e-

Charge carrier in semiconductor: e-, holes

Summary of Last LectureAcceptor (e.g. B in Si):

p-type Si(extra holes

=majority charge carriers)

Donor (e.g. P in Si):n-type Si

(extra electrons= majority charge carriers)

Conduction band

Valence band

Semiconductor

Eg ~ 1 eVextra

Levelsintroduced

Conduction band

Valence band

Semiconductor

Eg ~ 1 eV

Doping

Summary of Last LectureSemiconductor Junction: np

• Rectifying diode• Initial diffusion of holes

towards the n-region and e-towards the p-region [a), b)]

• Charge build-up on either side of the junction [c)]

• p-region → negativen-region → positive [c)]

• Electric field gradient across the junction → process halted → region of immobile space charge [d)]

• Due to electric field: potential difference across the junction → contact potential V0 (~1V)

holes

e-

Region of changing potential:depletion zone or space charge region

Summary of Last LectureReversed bias junction

• pn junction:1) intrinsic electrical field not

intense enough to provideefficient charge collection

2) thickness of depletion zoneonly sufficient for stoppinglowest energy particles

• apply reverse bias voltage to junction, i.e. a negative voltage on the p-side:→ attracts holes in the p-region

( “ e- “ “ n-region)→ enlarge depletion zone→ “ sensitive volume

1. d ~ √Vb2. the higher Vb, the

higher the charge collection efficiency

Summary of last lecturePhysics – device characteristics

• In general: depletion zone extends further into the light doped side

• Depletion zone• Contact/depletion

voltage• Resistivitiy

qNeffd V biasε2= ☼

ε2

2DNV

qefffd =

)(1

NN ppnnq µµ

ρ+

=

=µ he,=Neff

electron, hole mobility

Effective carrier concentration

x = distance from junction D = silicon thickness

Detector characteristics of Semiconductors

Basic configuration for operatinga junction diode as a radiation detector:Electrodes must be fitted on theTwo sides of the semiconductor

But: remember surface barrier diodes:Depositing metal directly onto the Semiconductor material results in the Creation of a rectifying junction with a Depletion zone ext. into the semiconductor Using heavily doped layer of

n+ and p+ between metal andsemiconductor solves the problem (see equation ☼ →d ~ 0)

What’s needed: Ohmic contact (a metal-semiconductorcontact with a linear or near linear current voltage characteristic)

Average Energy per Electron-Hole Pair

• The average energy for creating a electron-hole pair at a given temperature is independent of the type and energy of the radiation and only dependent on the material

• Number of charge carriers is almost an order of magnitude higher than in gases

• Compared to the number of photo electrons in a scintillation counter the number of charge carriers are almost two orders of magnitude higher→ greatly improved energy

resolution

2.96 eV3.81 eV77 K

3.62 eV300 K

GeSi

Energy gaps are only ~ 1 eV

Where do the other two thirdsOf the energy go?

Lattice vibrations

Linearity• If depletion region is sufficiently thick to completely stop all

particles → semiconductor response ~ E• If E energy of radiation E/w electron-hole pairs should be

produced, w average energy from table before• Voltage observed at the electrodes:

V = Q/C = n E/ (w C), C capacitance of semiconductor, w average energy, n charge collection efficiency → V ~ E

• w independent of type of radiation → E independent of type of radiation (only true for lightly ionizing particles such as electrons and positrons, plasma effects occur for heavier ions)

If depletion zone is smaller → nonlinear response (not the fullEnergy is deposited);Instead the energy loss ∆E is measured, which is a non-linear Function of energy

Leakage current• Reversed bias diode is ideally nonconduction, but a

small fluctuating current flows through the semiconductor when a voltage is applied → noise

• Sources of leakage current:– Movement of minority charges (e.g. holes from the n-region

attracted across the junction to p-region), ~ nA/cm2

– Thermally generated electron-hole pairs originating from the recombination and trapping centers in the depletion region (depends on the absolute number of traps), ~µA/cm2

– Leakage current through the surface channels (depends on very many factors, e.g. the surface chemistry, contaminants, surrounding atmosphere, etc.)minimize by clean encapsulation

Sensitivity and Intrinsic Efficiency

• Intrinsic detection efficiency close to a 100% for charged particles (as very few particles will fail to create some ionization in the sensitive volume)limiting factors: leakage current in the detector and noise from the associated electronics →sets lower limit on the pulses which can be detected

• Ensure adequate signal: depletion depth sufficiently thick → S (ioniz) > S (noise)

• Energy measurements: d > particle range

Sensitivity

Leo, page 230

Gamma Ray Detection

• Germanium is preferred over silicon because of higher Z

• However because of the smaller band gap, leakage current in Ge to high at room temperatures → Ge must be cooled (liquid nitrogen)

• For low energies below ~ 30 keV silicon detectors are preferred because the K-edge in Ge is located at ~11 keV

Comparison of NaI detector and Ge detector for Co-60

Energy Measurement:Energy resolution: # of information carriers:

Scintillator: > 100 eV / photon Gas: 20 – 40 eV / chargeSilicon: 3.6 eV / chargeGermanium: 2.8 eV / charge

Silicon advantages:• low ionization energy (good signal)• long mean free path (good charge collection efficiency)• high mobility (fast charge collection)• low Z (low multiple scattering)• production technologies very well developedGermanium: higher Z detect γ radiation (needs cooling…)

Diffused Junction Diodes

• Among the first fabricated• Produced by diffusing n-type impurities such

as phosphorus into one end of a homogeneous p-type semiconductor at high temperatures (~1000 deg C)

• By adjusting concentrations and diffusion time → junctions lying at a depths of few tenths of a microns to two microns can be produced

Ion-Implanted Diodes

• Formed by bombarding the semiconductor crystal with a beam of impurity ions from an accelerator

• By adjusting the beam energy, the impurity concentration and depth profile can be controlled

• Since radiation damage is incurred, the semiconductor must be annealed at temperatures of 500 deg C

Surface Barrier Diode

See previous lecture

Position Sensitive Detector

Micro-Strip Detectors

Applications

• CCD detectors• APS detectors• Micro vertex detectors• More about those later